Recently, electronic control fuel injection devices have been increasingly installed as fuel supplying devices, instead of conventional carburetors, in accordance with requirements for emission control and reduced fuel consumption. Plungers of solenoid valves in such electronically controlled fuel injection devices, hydraulic apparatuses, and various kinds of machining apparatuses are essentially required to have high alternating-current magnetic properties for responsiveness, strength (wear resistance) for resisting frequent impacts with a partner member, and corrosion resistance in the environment. Magnetic members for automobiles are also essentially required to have stable magnetic properties in a temperature range from about −40 to 200° C. which are encountered in practice in the environment.
Soft magnetic stainless steels have been used as materials for magnetic members in the above fuel injection devices and the like, since corrosion resistance and magnetic properties are important, and the members have been manufactured by mechanical forming method such as plastic working and machining methods, as shown in Japanese Examined Patent Application Publication No. 5-10419. However, the magnetic members such as electronic fuel injection valves for automobiles have complicated shape and are required to have high dimensional accuracy, whereby machinability, corrosion resistance and magnetic properties cannot be simultaneously improved together, and the manufacturing costs are high.
In order to solve the above problems, Japanese Unexamined Patent Application Publications Nos. 7-179983 and 2002-275600 propose production methods using powder metallurgy methods. Japanese Unexamined Patent Application Publication No. 7-179983 discloses a production method for soft magnetic sintered material in which a mixed powder of an Fe—Cr alloy powder, an Fe—Si alloy powder, and an Fe powder and mixed powder of an Fe—Cr—Si powder and an Fe powder are compacted and sintered. Japanese Unexamined Patent Application Publication No. 2002-275600 discloses the use of a powder as a raw material made by granulating a fine stainless steel powder and a fine Si powder or a fine Fe—Si powder.